He ran over Myrtle like you'd run over a dog and never even stopped his car."

Compared to human beings, dogs are subservient and “lower on the food chain.” Running over Myrtle like a “dog” suggests that it was an easy thing for Gatsby to do because Myrtle’s life is somewhat meaningless in the overall social hierarchy.

In “The Yachts” by William Carlos Williams, there is a sea full of dead bodies that the “skillful yachts pass over.” This serves as an allegory for social class and how wealthy or more prominent people seem to float by easily, while those without these privileges struggle and suffer.

In The Great Gatsby, the car fills the same role as the yachts in the poem. Tom is still unaware that it was Daisy, not Gatsby, who ran over Myrtle, but he is saying the person who did it felt some kind of sick entitlement to kill another person without looking back twice. Myrtle is compared to a lowly dog because those around her don’t have enough consideration for her as a member of society. Myrtle is collateral damage in the quest for the wealthy to get ahead in society and take care of their own needs.

The irony here is thick: Tom is used to “running people over” (metaphorically) in his quest to satisfy his own needs, and he, like Daisy, has now actually gotten somebody killed. Nick says as much below: “They were careless people, Tom and Daisy—they smashed up things and creatures…”